The invention relates to a color display tube having an electromagnetic deflection unit coaxially surrounding the display tube, which unit comprises a field deflection coil system and a line deflection coil system, the line deflection coil system having two diametrically arranged flared line deflection coils of the saddle type with a front flange at the widest end and with two longitudinal conductors packets extending symmetrically relative to the axis of the display tube.
Increasingly strict requirements are imposed on the performance of color display tubes having electromagnetic deflection units particularly when they are used in monitors. Stringent requirements are imposed, for example on the shape of the raster.
In conventional TV receiver sets or in monitor sets a raster is formed by causing an electron beam to scan the front plate of the display tube. The (geometrical) raster errors which may occur are north-south raster errors (errors on the upper and lower side of the raster) and east-west raster errors (errors on the right and left-hand side of the raster). In color display tubes having an in-line arrangement of the electron guns the north-south raster errors becomes manifest as a certain waviness of the upper and lower edge of the raster ("second harmonic distortion"). To reduce this error, it is known from U.S. Pat. No. 4,229,720 to give the front flange of each line deflection coil, which flange is transverse to the axis of the display tube, the shape of a polygon. The vertices are positioned in such a way that field components are generated when energizing the line deflection coil, which components reduce the north-south raster error. This solution was later found to have the drawback that the polygonal flange shape gives rise to loss of energy on the one hand and generates unwanted radiation on the other hand. An advantage of the (polygonal) front flange transverse to the axis of the display tube is, however, that it provides a reference plane in the axial (z) direction when mounting the line deflection coil, so that an accurate adjustment of the z position which is required for a satisfactory color purity, is possible.